Each
nerve cell consists of a cell body with a prominent nucleus. There are fine
projections mainly of two types extending from the cell body of the nerve cell.
The small projections are dendrites while a long one that extends to different
pans of our body is called Axon. The axon is surrounded by a specialized
insulator sheath called myelin sheath. This sheath is interrupted at regular
intervals called nodes of ranvier. The myelin sheath is made up of Schwann
cells and chiefly consists of fatty material. Axons not having the sheath are
non-myelinated.
Nerve
cell is the structural and functional unit of nervous system. Our nervous
system consists of around about 10 billion of them, which communicate with each
other in a specific manner. Dendrites of one nerve cell connect to the other or
to the axons of the other nerve cell through connections called as a ‘synapse’.
Afferent neurons or Sensory neurons :
Afferent
(or ferrying towards) which carry messages towards the central nervous system
(spinal cord or brain) from nerve endings on the muscles of different sense
organs that sense the change in surroundings are called stimulus detectors.
These are also called ‘sensory’ nerves.
Efferent neurons or motor neurons:
Efferent
(or ferrying away) which carry messages from the central nervous system to
parts that shall carry out the response or the effectors (nerve endings). They
are also called ‘motor’ nerves.
The reflex arc
Fore Brain:
Offactory lobes : These clubshoped widely spread bodies
are visible from the ventral surface only.
Functions: These are concerned with sense of
smell.
Cerebrum : it consists of two lobes called
cerebral hemispheres. The surface of the cerebrum has many folds, they appear
as elevations (Gyri) and depressions (Sulci. Sulci are very prominent and
divide each hemisphere into four lobes.
Functions :
i)
Seat of mental abilities, controls thinking, memory, reasoning, perception,
emotions and speech.
ii)
Interprets sensations and responds to cold, heat, pain and pressure.
Diencephalon : This rhomboidal shoped lobe visible
from imferior surface of brain. It lies between cerebrum and mid brain. It is
divided into thalamus and hypothalamus.
Functions :
i)
Relay centre for sensory impulses, such as pain, temperature and light.
ii)
Reflex centre for muscular activities.
iii)
Centre for certain emotions such as anger.
iv)
Centre for water balance, blood pressure, body temperature, sleep and hunger.
v)
The hypothalamus controls the pituitary gland, which functions as the master
gland.
Mid-brain : This is a small thick stalked portion.
This connects forebrain with cerebellum and pons of hind brain.
Functions: It relays motor impulses from the
cerebral cortex to the spinal cord and relays sensory impulses from the spinal
cord to the thalamus, reflexes for sight and hearing.
Hind brain
Cerebellum: It is located below the cerebrum and
above medulla oblongata. It consists of two large cerebellam hemispheres.
Functions:
i)
Maintains posture, equilibrium and muscle tone.
ii)
Coordinates voluntary movements initiated by cerebrum.
Medulla oblongata: It is almost triangular shaped.
Medulla oblongata extends from pons to spinal cord. Ask your teacher grey and
white matters of medulla oblongata
Functions:
i)
Contains centre for cardiac, respiratory and vasomotor activities. (Vasomotor
refers to actions upon a blood vessel which alter its diameter)
ii)
Coordinates reflexes like swallowing, coughing, sneezing and vomiting.
Spinal Cord
Autonomous nervous system
You
know that medulla oblongata is the region that regulates heartbeat, breathing
etc. the system that helps to bring about such activities of internal organs is
called autonomous nervous system. Normally such involuntary activities take
place by the coordinated efforts of the medulla oblongata and autonomous
nervous system.
Ganglia
near the vertebral column are connected to the spinal cord by nerves. The
sympathetic system is formed by the chain of ganglia on either sides of the
vertebral column and the associated nerves. The parasympathetic system is
formed by the nerves arising from the ganglia of the brain and the posterior
part of the spinal cord. These together constitute the autonomous nervous
system. It is the part of the peripheral nervous system consisting of twelve
pairs of cranial nerves and thirty one pairs of spinal nerves.
Harmones
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Users
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Abscisic acid
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Closing of stomata; seed dormancy
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Auxins
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Cell elongation and differentiation
of shoots and roots
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Cytokinins
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promote cell division, promotion of
sprouting of lateral buds, delaying the ageing in leaves, opening of stomata.
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Ethylene
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Ripening of fruit
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Gibberellins
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germination of seeds and sprouting of
buds; elongation of stems;
stimulation of flowering; development
of fruit, breaking the dormancy
in seeds and buds.
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